If the browser is the bottleneck, why doesn't someone just
'make it happen' on one of the many open source browser
platforms?

Because changing one of the many platforms wouldn't make anything happen.
Content publishers move at the pace of the slowest. Youv'e got to change all
the platforms to have any effect.

So the problem is not the browser but the whole browser / HTML
ecosystem , which is stacked against any alternative, XML included.
So the question I guess is how would XML (or anything else) deliver a
compelling advantage that could challenge that HTML stack, as opposed
to existing in niches of that stack (which appears to be the case for
XML today, in the Web/HTML context).